Monitoring circuit for a supply voltage

Abstract

The subject of the invention is a monitoring circuit for at least one supply voltage, having comparators that generate a first report signal if the supply voltage is below a predeterminable threshold value and a second report signal after a fixed warning time elapses. The supply voltage is supplied in a first circuit unit (4) to a first comparator (7). The first comparator is followed by a switch element, with which a capacitor (19), connected parallel to it and chargeable from an operating voltage source, can be short-circuited in order to lower the input voltage of a second comparator (13), disposed in the control unit (4), to below a second reference voltage that defines a limit value for the resetting of the report signal. The noninverting output of the second comparator (13) is followed by the base of an output transistor (14), whose emitter is connected to the negative pole of the operating voltage, the positive pole whereof is connected via a resistor (31) to the collector of the output transistor (14). The first report signal appears at the output transistor (14). The collector of the output transistor (14) is followed via a diode (20) by a capacitor (23), which in a second circuit unit (24) identical to the first circuit unit (4) is connected to the first comparator (7').

We claim: 
     
       1. A monitoring circuit for at least one supply voltage, having comparators that generate a first report signal if the supply voltage is below a predeterminable threshold value and a second report signal after a fixed warning time elapses, wherein the supply voltage is delivered in a first circuit unit (4) to a first comparator (7) for comparison with a reference voltage that defines a lower, first limit value; that in the first circuit unit (4) the first comparator (7) is followed by a switch element, with which a capacitor (19), connected parallel to the switch element and connected to an input of a second comparator (13) and chargeable from an operating voltage source, can be short-circuited by the switch element in order to lower the input voltage of the second comparator (13), disposed in the first circuit unit (4), to below a second reference voltage that defines a limit value for resetting the first report signal; that the noninverting output of the second comparator (13) is followed by the base of a bipolar output transistor (14), whose emitter is connected to the negative pole of the voltage source that supplies operating voltage to the control unit (4), the positive pole of the voltage source being connected via a resistor (31) to the collector of the output transistor (14), at which, or at least at an amplifier following the output transistor (14), the first report signal appears; that the collector of the output transistor (14) is followed via a diode (20) by a second capacitor (23) that can be charged by a second resistor (21) and discharged by a third resistor (22) and that is connected in a second circuit unit (24), identical to the first circuit unit (4), to the first comparator (7') of the second circuit unit; and that the second report signal is generated at the collector of the output transistor (14') of the second circuit unit (24) or at least at an amplifier following this output transistor.     
     
     
       2. The monitoring circuit of claim 1, wherein the supply voltage is the same voltage as the operating voltage of the first and second circuit units (4, 24).     
     
     
       3. The monitoring circuit of claim 1, wherein the switch elements in the first and second circuit units (4, 24) are thyristors (10, 10') that can be turned on and off.     
     
     
       4. The monitoring circuit claim 1, wherein the collector of the output transistor (14) of the first circuit unit (4) is connected to a resistor (32), which is connected to the base of a bipolar transistor (33) and whose collector supplies the base of a further transistor (34) that forms one output of the monitoring circuit for the first report signal and whose emitter is connected via a resistor (38) to the positive pole of the operating voltage source and whose collector is connected via a resistor (30) to the negative pole of the operating voltage source.   
     
     
       5. The monitoring circuit of claim 4, wherein the collector of the output transistor (14') of the second circuit unit (24) is connected to a gate circuit (42), of which a further input is applied via a short-circuitable resistor (55) to the positive pole of the operating voltage; that the gate circuit (42) is connected on its output side via a resistor (32) to the base of a transistor (33'), whose base is supplied via a further resistor (50') by the positive pole of the operating voltage source and whose collector supplies the base of a further transistor (34), which forms one output of the monitoring circuit at its collector for the second report signal; and that the collector and the emitter of the further transistor (34) are connected each via a respective resistor (38', 39') to the positive and negative poles, respectively, of the operating voltage.   
     
     
       6. The monitoring circuit claim 5, wherein the bases of the further transistors (34, 34') that supply the outputs of the monitoring circuit are each connected via a respective resistor (40, 40') to the positive pole of the operating voltage source and via a further respective resistor (35, 35') to the cathode of a Zener diode (36), which is disposed in series with a resistor (37) between the poles of the operating voltage source.   
     
     
       7. The monitoring circuit claim 5, wherein the emitters of the further transistors forming the outputs of the monitoring circuit are each connected to a respective current limiting circuit.   
     
     
       8. The monitoring circuit claim 6, wherein the inverting outputs of the second comparators (13, 13') of the circuit units (4, 24') are each followed by transistors, whose emitters are applied to the positive pole of the operating voltage and whose collectors, each via a respective resistor (52, 52'), are connected to the base of a transistor (53, 53'), whose collector-to-emitter path can be connected parallel to the resistor (39, 39') connected to the emitter of the further transistor (34, 34') that supplies the output of the monitoring circuit.